[amsat-bb] Re: Polarity questions
angus at young5769.freeserve.co.uk
Sat Sep 20 22:57:46 PDT 2008
Hmmmmm might get shot down in flames for this but.
If you want to know the velocity factor of a coax (known or un-known) try
getting two eaqual lengths of coax which are physically 1/4wavelength long
i.e. 50cms for 2mtrs.
Short out ONE end (join the inner and outers together with a short a link as
possible) and do this for both pieces of coax.
We are going to make a coaxial dipole so these two shortened ends go on to a
piece of feeder (try and keep this feeder as short as possible also but the
dipole needs to be somewhere clear but does not to be 100 feet up either).
Yes you will need to support the dipole elements as they will just hang down
so perhaps a piece of garden cane or some other RF invisible material.
Trim back both OPEN ends of the dipole until you get the lowest possible
VSWR (on the centre frequency you want to use) and you should have very
close to an electrically 1/2wave dipole for the coax you used (each side of
the dipole will be a 1/4 wave long). You can use these figures for making a
1/4wave delay line or if your making a polarity control box you will need to
know the lengths of both 1/2 and 1/4 of the coax you are going to use, the
dipole will be considerably shorter than a normal wire dipole for the same
frequency because we have used the velocity factor to help reduce its
If you think that the feeder you have used is radiating either do perhaps 6
turns around a small 1" tube to make a balun at the feedpoint or alter the
length of the feeder slightly, if theVSWR alters when you change the length
of the feeder then the feeder is radiating.
My two pennys worth, should help you get very close to the figures you need
without the use of a GDO or an antenna analyser.
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